1. Field of the Invention
The present invention relates to lithographic projection apparatus and, more particularly, to an actuating device for controlling a magnetic force.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion or target field of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask (i.e., reticle), may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target field (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions or fields that are successively exposed. Known lithographic apparatus include so-called “steppers,” in which each target field is irradiated by exposing an entire pattern onto the target field in one sweep, and so-called “scanners,” in which each target field is irradiated by scanning the pattern through the projection beam in a given direction (e.g., the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion/field”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool.
Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.
The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target field of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target field of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target field, such as an integrated circuit.
Patterning devices may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
A support structure supports (i.e. bares the weight), of the patterning device. It holds the patterning device in a way depending on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as, for example, whether or not the patterning device is held in a vacuum environment. The support can be using mechanical clamping, vacuum, or other clamping techniques, for example electrostatic clamping under vacuum conditions. The support structure may be a frame or a table, for example, which may be fixed or movable as required and which may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.”
The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.”
The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.”
The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure.
The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index (e.g. water), so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.
Within a lithographic projection apparatus, supports are required to provide a permanent force to oppose gravity. For instance, quasi-static supports are required to support an isolated reference frame (which supports the projection system and various sensor devices) and isolate it from external vibrations. Dynamic supports are, for instance, required to support a short-stroke module for a substrate or patterning means on a long-stroke module. In such dynamic supports, a static force component is provided to support the weight of the short-stroke module and a dynamic force component is provided to drive the short-stroke module. In both static and dynamic supports, it is desirable that the support has very low stiffness to prevent the transmission of vibrations.
Previously, it has been proposed to provide a supporting force by means of magnetic attraction and/or repulsion such as, for instance, as disclosed in EP 1,001,512 or U.S. Pat. No. 5,780,943. However, the proposed solutions provide a supporting force that may be positional dependent both along and perpendicular to the support direction. The proposed solutions may also be subject to demagnetization effects.
The support using magnetic force as noted above, is further referred to as an magnetic actuator that provides a magnetic force. Consistent with the common ordinary understanding of the term “magnetic actuator,” which means a mechanical device that employs magnetic force to move or control something, the magnetic actuator serves to support a load, which is to be supported and/or positioned at a well-specified position. Typically, the magnetic actuator generates an adjustment force to adjust a position or a compensation force to counterbalance a required force (e.g. gravity), or both.
In the prior art, the actuator, commonly referred to as a Lorentz-type, uses a magnetic force to keep a load at a well-specified position, or to adjust that position due to a change of the actual load. The generation of the magnetic force by this type of actuator is based on the principle given by Lorentz for the relation between a charged particle, it's movement, and an external magnetic field.
Disadvantageously, during the actual operation of the actuator, such an actuator uses an electric current in a conducting coil to generate the magnetic force and at the same time creates a continuous heat dissipation. The heat dissipation may cause a temperature change in the system portion in which the actuator is located. The stability of the support may be adversely influenced by thermal drift, thermal expansion, and/or thermal stress due to temperature changes induced by the heat dissipation.
Furthermore, prior art magnetic actuators, used for achieving magnetic levitation of objects, the matter is more complicated since compensating gravity constantly during movement of the object under levitation, requires a continuous change of the amplitude of the current creating the magnetic field. An increase of that current may cause a demagnetization effect in the actuator. Moreover, the dissipation for creating a levitation effect will be relatively large and may cause thermal problems with other parts near the Lorentz actuator(s). Also, during levitation, relatively high accelerations may occur in the mechanical parts of the actuator which may cause large disturbance forces and possibly, related damage in those mechanical parts.
Another type of device used to generate adjustment forces, is an actuator based on the piezoelectric principle, wherein the actuator comprises a piezoelectric crystal and a electrically induced displacement in a piezoelectric crystal is applied to change a position of the actuator. Although their dissipation is small compared to that for Lorentz-type actuators, piezoelectric actuators disadvantageously have a relatively small actuating range, due to the limited value of the piezoelectric effect. Also, piezoelectric actuators are not suitable for creating levitation of an object.
In a continuous effort to create lithographic projection apparatuses with a capability to define patterns with increasingly smaller features, the wavelength of the radiation beam has reduced to increasingly lower values. At present, a typical wavelength is 157 nm, which is in the (deep) ultra-violet part of the electromagnetic spectrum (UV). It is noted that a smaller wavelength in the UV range is possible (e.g., 126 nm) or in the extreme ultra-violet (EUV) in the range of 5-20 nm. With this said, it has been observed that the mechanical and thermal stability of the lithographic projection apparatus must be controlled so that no influence of heat dissipation by actuators is detected in the performance of the apparatus. By going to increasingly lower values of feature sizes to be exposed and by going to increasingly lower values of the radiation wavelength of the lithographic projection apparatus, the requirements for adjusting actuators with better thermal and mechanical stability increase accordingly.